An aircraft-internal air conditioning system requires, for proper functioning, highly compressed process air which is provided to the air conditioning system by the engines or the auxiliary power unit (APU) of the aircraft or when the aircraft is on the ground also by an aircraft-external high-pressure air generation unit. If none of these process air supply systems is available, the aircraft air conditioning system cannot be operated. Moreover, when an aircraft is on the ground, for example during prolonged maintenance work, an operation of the aircraft air conditioning system may not be desirable for cost reasons or not be possible for safety reasons. It is therefore customary to supply aircraft when on the ground with precooled air which is provided by an aircraft-external low-pressure air generation unit.
Low-pressure air generation units are available in different designs and with different cooling capacities. Most low-pressure air generation units used at present provide cooling air at a temperature of approx. +5° C., the generated cooling air quantity as well as the system pressure and thus the pressure of the cooling air in each case depending on the design of the low-pressure air generation units. An aircraft-external low-pressure air generation unit is connected by connecting hoses to standardised aircraft-side connections, so that the cooling air generated by the low-pressure air generation unit can be led into the interior of the aircraft. There the cooling air is led, with the aid of aircraft-internal air distribution systems, into the aircraft regions to be cooled, such as e.g. the passenger zone, the cockpit, the cargo compartments or various installation spaces of heat-generating, in particular electronic components of the aircraft. Not until the aircraft engines are about to be started is the aircraft-external low-pressure air generation unit separated from the aircraft again and the cooling of the aircraft regions to be cooled taken over by the aircraft air conditioning system.
A currently used aircraft air conditioning system comprises a mixing chamber which is arranged in a pressurised region of the aircraft and, in the operation of the aircraft air conditioning system, is fed with very cold air at a temperature of up to approx. −25° C. and a relatively high pressure as well as warm circulating air at a temperature of approx. +30° C. by the air conditioning units of the aircraft air conditioning system. The air mixed in the mixing chamber for setting a desired temperature is subsequently led via various air distribution lines into the aircraft regions to be cooled. To control the air flow within the air conditioning system, lines connecting the air conditioning units to the mixing chamber have in each case corresponding check valves arranged in them, which prevent circulating air provided for feeding into the mixing chamber from escaping into the air conditioning units arranged outside the pressurised aircraft region.
If, when the aircraft is on the ground, an aircraft-external low-pressure air generation unit takes over the cooling of the aircraft regions to be cooled, the air provided by the low-pressure air generation unit is led directly into the mixing chamber of the aircraft air conditioning system via a corresponding aircraft-side low-pressure air distribution system. The low-pressure air distribution system comprises an inlet line which extends through the unpressurised belly fairing into the pressurised region of the aircraft towards the mixing chamber. A check valve arranged in the inlet line in a transition region between the pressurised and the unpressurised region of the aircraft prevents a backflow of air from the portion of the inlet line arranged in the pressurised region of the aircraft into the inlet line portion extending through the unpressurised aircraft region. In the event of a leakage in the part of the low-pressure air distribution system arranged in the unpressurised aircraft region, the check valve thus prevents air from escaping from the pressurised region of the aircraft.
With aircraft cooling systems known at present, the cooling of the aircraft regions to be cooled is taken over either exclusively by the aircraft air conditioning system or exclusively by the aircraft-external low-pressure air generation unit. However, a simultaneous operation of both systems is not possible. In other words, the mixing chamber of the aircraft air conditioning system can be fed either with circulating air and cold air generated by the aircraft-external low-pressure air generation unit or with circulating air and cold air generated by the air conditioning units of the aircraft air conditioning system. An aircraft-internal circulating air conveying device, such as e.g. a circulating air fan, can thus be operated either together with the aircraft-external low-pressure air generation unit or together with the air conditioning units of the aircraft air conditioning system, but not together with the aircraft-external low-pressure air generation unit and the air conditioning units of the aircraft air conditioning system.
This is due to the different system pressures of the aircraft air conditioning system on the one hand and the aircraft-external low-pressure air generation unit on the other hand. The air conditioning units of the aircraft air conditioning system work at a relatively high system pressure. In comparison, the system pressure provided by a conventional low-pressure air generation unit is usually significantly lower. Consequently, owing to the check valves provided in the system lines, it would not be possible to ensure stable operation of the entire cooling system with a simultaneous operation of the aircraft air conditioning system and the aircraft-external low-pressure air generation unit. Instead, “rattling” and eventually damage of the valves would occur. Moreover, the low-pressure air generation units currently used are not capable of supplying a system pressure which, with a joint operation of the aircraft air conditioning system and an aircraft-external low-pressure of air generation unit, ensures reliable opening of the check valve which is arranged in the inlet line of the low-pressure air distribution system connecting the low-pressure air generation unit to the mixing chamber of the aircraft air conditioning system.
In order to enable a simultaneous operation with an aircraft air conditioning system, an aircraft-external low-pressure air generation unit would thus have to work at a significantly higher system pressure than is the case today. However, this would require not only replacement of the air generation units used at present, but also adaptation of the aircraft-internal low-pressure air distribution system, since this system is at present not designed for an increased operating pressure. Moreover, it would possibly be necessary to replace the check valves used in the entire cooling system with actively driven valves. Finally, an increased pressure in the mixing chamber would possibly affect the operation of the circulating air fans conveying the circulating air into the mixing chamber.
Aircraft cooling systems which cool the aircraft regions to be cooled either exclusively by means of the aircraft air conditioning system or exclusively by means of an aircraft-external low-pressure air generation unit have, however, the disadvantage that the aircraft air conditioning system must be dimensioned so that when the aircraft is on the ground a proper cooling of the aircraft regions to be cooled is ensured also on very hot days. The aircraft air conditioning system is therefore relatively powerful, which increases the system weight and thus the fuel consumption of the aircraft. Moreover, existing aircraft air conditioning systems are designed so that they can ensure a specific temperature in the aircraft regions to be cooled, at specific aircraft ambient temperatures. Consequently, lower temperatures in the aircraft regions to be cooled, at constant or higher ambient temperatures, or constant temperatures in the aircraft regions to be cooled, at higher ambient temperatures, could only be achieved by adaptation, i.e. enlargement, of the existing aircraft air conditioning system. However, this would lead to extra costs, extra weight as well as possibly problems with system integration. Finally, in an aircraft air conditioning system, the air conditioning units and the circulating air fans must be operated at maximum power to provide the maximum cooling output. This results in high energy consumption.